Specification of fvwm variables
-------------------------------

Anything that looks like $[VAR] in fvwm command is a variable and a subject
for expanding. There are 2 kinds of variables, user defined and built-in.

If the requested variable MY_VAR is not defined, it is not expanded in
the command, i.e. the literal text "$[MY_VAR]" is left as is.

User defined variables
----------------------

A user may manage his variables using SetEnv and UnsetEnv commands,
and query them using Test command options EnvIsSet and EnvMatch.

All shell variables that were defined on the fvwm startup are available,
like $[HOME]. New invoked processes will inherit all these variables
plus all new ones added by SetEnv. User variables containing non English
alphanumeric characters, like "-" or "+", are not inherited by invoked
processes, this may be used to create variables private to fvwm. Examples:

  SetEnv USER nobody         # change existing variable
  SetEnv my_name "Some One"  # create a new variable
  SetEnv MY-NAME "Some One"  # only accessible in fvwm process, $[MY-NAME]

Any variable name starting with a digit or containing a period is
reserved by fvwm.

Built-in variables
------------------

These are read-only variables set by fvwm, like $[cs.fg2] or $[w.class]
or $[desk.n] or $[screen] or $[gt.some text] or $[.] or $[version.line].

Positional variables
--------------------

These are variables available in functions only, they contain function
parameter, like $[1] or $[12] (starting from 1). Additionally, $[*]
contains the parameters text on the function invocation line (as is).

It makes sense to support syntax $[2-4] (three parameters starting from
the second) or $[2-] (all parameters starting from the second).

Variable filters
----------------

By default, any variable $[VAR] is enclosed into single quotes and any
single quote in it is escaped with a backslash. The exception is
positional variables $[<number>]. Traditionally, variables like $[2] are
replaced with their content without any quoting, by default at least.
Another exception is $[*], also for backward compatibility. Examples:

  Next Exec xmessage $[w.name]
  Next Exec xmessage "Window name is $[w.name]"

The last string is expanded to "Window name is 'FVWM Home Page - Firefox'".

It's possible to change the default quoting by specifying explicit filters:

  $[VAR]             - expanded to: 'My %^&* window'
  $[VAR:quote]       - expanded to: 'My %^&* window'
  $[VAR:dquote]      - expanded to: "My %^&* window"
  $[VAR:noquote]     - expanded to: My %^&* window
  $[VAR:glob]        - expanded to: 'My %^&** window'
  $[VAR:menu]        - expanded to: 'My %%^&* window'

More than one filter may be applied if needed, for example:

  AddToMenu WindowOps 'Go to first desk, $[desk.name0:menu:dquote]'

It makes sence to allow a raw-filter for positional parameters, which is the
one used by default for $[*].

Escaping
--------

A backslash may be used to quote special chars. These are not variables:

  Echo \$[VAR]
  Echo $\[VAR]

Here a colon is literal and does not denote filters:

  Echo $[gt.Phone\:]